Hot-rolled steel sheet and method for manufacturing same

Abstract

A hot-rolled steel sheet has, as a chemical composition, by mass %: C: 0.01% to 0.30%; Si: 0.01% to 3.00%; Mn: 0.20% to 3.00%; P: 0.030% or less; S: 0.030% or less; Al: 0.001% to 2.000%; N: 0.0100% or less; and Ni: 0.02% to 0.50%, in which among measurement points at which elemental analysis is performed at a measurement pitch of 1 μm using an EPMA in a region of 250 μm×250 μm on a surface, the percentage of measurement points having a Ni content of 0.5 mass % or more is 10% to 70%.

What is claimed is: 
     
       1. A hot-rolled steel sheet comprising, as a chemical composition, by mass %:
 C: 0.01% to 0.30%; 
 Si: 0.01% to 3.00%; 
 Mn: 0.20% to 3.00%; 
 P: 0.030% or less; 
 S: 0.030% or less; 
 Al: 0.001% to 2.000%; 
 N: 0.0100% or less; 
 Ni: 0.02% to 0.50%; 
 Nb: 0% to 0.060%; 
 V: 0% to 0.20%; 
 Ti: 0% to 0.20%; 
 Cu: 0% to 0.20%; 
 Cr: 0% to 0.20%; 
 Mo: 0% to 1.00%; 
 B: 0% to 0.0020%; 
 W: 0% to 0.50%; 
 Mg: 0% to 0.010%; 
 Ca: 0% to 0.0100%; 
 REM: 0% to 0.0100%; 
 O: 0% to 0.0100%; 
 Zr: 0% to 0.500%; 
 Co: 0% to 0.500%; 
 Zn: 0% to 0.500%; 
 Sn: 0% to 0.500%; and 
 a remainder consisting of Fe and impurities, 
 wherein among measurement points at which elemental analysis is performed at a measurement pitch of 1 μm using an Electron Probe Micro Analyzer in a region of 250 μm×250 μm on a surface, a percentage of measurement points having a Ni content of 0.5 mass % or more is 10% to 70%. 
 
     
     
       2. The hot-rolled steel sheet according to  claim 1 ,
 wherein the chemical composition contains one or two or more selected from: 
 Nb: 0.003% to 0.060%, 
 V: 0.01% to 0.20%, 
 Ti: 0.01% to 0.20%, 
 Cu: 0.01% to 0.20%, 
 Cr: 0.01% to 0.20%, 
 Mo: 0.01% to 1.00%, 
 B: 0.0005% to 0.0020%, 
 W: 0.01% to 0.50%, 
 Mg: 0.001% to 0.010%, 
 Ca: 0.0010% to 0.0100%, 
 REM: 0.0010% to 0.0100%, and 
 O: 0.0005% to 0.0100%. 
 
     
     
       3. The hot-rolled steel sheet according to  claim 2 ,
 wherein the chemical composition contains 
 Si: 0.50% to 3.00%. 
 
     
     
       4. The hot-rolled steel sheet according to  claim 2 ,
 wherein the chemical composition contains 
 Si: 0.01% or more and less than 0.50%, and 
 Al: 0.050% to 2.000%. 
 
     
     
       5. The hot-rolled steel sheet according to  claim 2 ,
 wherein the chemical composition contains 
 Si: 0.01% or more and less than 0.50%, and 
 Al: 0.001% or more and less than 0.050%, and 
 a total of Si and Al is 0.50% or more and less than 0.55%. 
 
     
     
       6. The hot-rolled steel sheet according to  claim 3 ,
 wherein among the measurement points at which the elemental analysis is performed on the surface, a percentage of measurement points having an O content of 0.5 mass % or more is 30% or less. 
 
     
     
       7. The hot-rolled steel sheet according to  claim 4 ,
 wherein among the measurement points at which the elemental analysis is performed on the surface, a percentage of measurement points having an O content of 0.5 mass % or more is 30% or less. 
 
     
     
       8. The hot-rolled steel sheet according to  claim 5 ,
 wherein among the measurement points at which the elemental analysis is performed on the surface, a percentage of measurement points having an O content of 0.5 mass % or more is 30% or less. 
 
     
     
       9. The hot-rolled steel sheet according to  claim 1 ,
 wherein the chemical composition contains 
 Cu: 0.01% to 0.20%, and 
 Ni/Cu is 0.50 or more. 
 
     
     
       10. The hot-rolled steel sheet according to  claim 1 ,
 wherein among the measurement points at which the elemental analysis is performed on the surface, an average interval between the measurement points having a Ni content of 0.5 mass % or more is 3 to 10 μm. 
 
     
     
       11. The hot-rolled steel sheet according to  claim 1 ,
 wherein the surface has a rust preventive oil film. 
 
     
     
       12. The hot-rolled steel sheet according to  claim 1 ,
 wherein the surface has a chemical conversion film. 
 
     
     
       13. The hot-rolled steel sheet according to  claim 2 ,
 wherein the chemical composition contains 
 Cu: 0.01% to 0.20%, and 
 Ni/Cu is 0.50 or more. 
 
     
     
       14. The hot-rolled steel sheet according to  claim 3 ,
 wherein the chemical composition contains 
 Cu: 0.01% to 0.20%, and 
 Ni/Cu is 0.50 or more. 
 
     
     
       15. The hot-rolled steel sheet according to  claim 4 ,
 wherein the chemical composition contains 
 Cu: 0.01% to 0.20%, and 
 Ni/Cu is 0.50 or more. 
 
     
     
       16. The hot-rolled steel sheet according to  claim 5 ,
 wherein the chemical composition contains 
 Cu: 0.01% to 0.20%, and 
 Ni/Cu is 0.50 or more. 
 
     
     
       17. The hot-rolled steel sheet according to  claim 6 ,
 wherein the chemical composition contains 
 Cu: 0.01% to 0.20%, and 
 Ni/Cu is 0.50 or more. 
 
     
     
       18. The hot-rolled steel sheet according to  claim 7 ,
 wherein the chemical composition contains 
 Cu: 0.01% to 0.20%, and 
 Ni/Cu is 0.50 or more. 
 
     
     
       19. A method for manufacturing a hot-rolled steel sheet comprising:
 heating a steel piece having the chemical composition according to  claim 1  in a heating furnace; 
 descaling the heated steel piece; and 
 hot-rolling the steel piece after the descaling to obtain a hot-rolled steel sheet, 
 wherein in the heating,
 after a surface temperature of the steel piece reaches 1,100° C. or higher, the steel piece is held for 60 minutes or longer under an atmosphere having an air ratio of 0.9 or more, and 
 an extraction temperature is 1,180° C. or higher, and in the descaling, 
 the steel piece of which the surface temperature is 1,170° C. or higher is descaled at least once with an injection pressure of 5 to 50 MPa, and 
 the surface temperature of the steel piece is held at 1,100° C. or higher for 20 to 240 seconds after completion of the descaling. 
 
 
     
     
       20. A hot-rolled steel sheet comprising, as a chemical composition, by mass %:
 C: 0.01% to 0.30%; 
 Si: 0.01% to 3.00%; 
 Mn: 0.20% to 3.00%; 
 P: 0.030% or less; 
 S: 0.030% or less; 
 Al: 0.001% to 2.000%; 
 N: 0.0100% or less; 
 Ni: 0.02% to 0.50%; 
 Nb: 0% to 0.060%; 
 V: 0% to 0.20%; 
 Ti: 0% to 0.20%; 
 Cu: 0% to 0.20%; 
 Cr: 0% to 0.20%; 
 Mo: 0% to 1.00%; 
 B: 0% to 0.0020%; 
 W: 0% to 0.50%; 
 Mg: 0% to 0.010%; 
 Ca: 0% to 0.0100%; 
 REM: 0% to 0.0100%; 
 O: 0% to 0.0100%; 
 Zr: 0% to 0.500%; 
 Co: 0% to 0.500%; 
 Zn: 0% to 0.500%; 
 Sn: 0% to 0.500%; and 
 a remainder comprising Fe and impurities, 
 wherein among measurement points at which elemental analysis is performed at a measurement pitch of 1 μm using an Electron Probe Micro Analyzer in a region of 250 μm×250 μm on a surface, a percentage of measurement points having a Ni content of 0.5 mass % or more is 10% to 70%.